This invention claims the benefit of Japanese Patent Application No. 2001-24471, filed on Jan. 31, 2001, which is hereby incorporated by reference.
1. Field of the Invention
This invention relates to a high pressure electric discharge lamp with a low starting voltage. Such an electric discharge lamp can find various applications including light sources of vehicles where a high voltage transformed from a battery voltage is applied to the lamp.
2. Description of the Related Art
Generally, high pressure electric discharge lamps including metal halide lamps that are typically used as headlamps of automobiles have a double tube structure formed by enclosing an arc tube that contains mercury, at least one metal halide, and starter gas with an outer tube made of a material absorbing ultraviolet rays.
Japanese Patent Laid-Open Publication No. Hei. 6-20645 discloses an electric discharge lamp having a double tube structure formed by fitting a double-end type arc tube to a base located only at an end thereof. The disclosed discharge lamp is highly vibration resistant and impact resistant by using a straight outer tube that does not surround the current feeding conductors. The current feeding conductors turn back toward the base from the sealing section located opposite to the base in order to make the outer tube surround the arc tube with only a narrow gap separating them. The outer tube does not need to be airtightly sealed and air is found in the space between the outer tube and the arc tube.
Metal halide lamps that are being popularly used as automobile head lamps automobile contain rare gas such as Xe gas as starter gas at about 7 atm to more than 10 atm at room temperature, because they are required to generate an effective flux of light immediately after a start. Thus, in such a lamp, the Xe gas can emit light immediately after a start and a high temperature arc rapidly heats the arc tube to accelerate the evaporation of mercury and metal halide to quickly reach a predetermined amount of luminous flux. Xenon lamps contain xenon gas that also operates as starter gas at about 20 atm in order to quickly reach a predetermined amount of luminous flux.
However, high pressure electric discharge lamps containing starter gas including xenon gas at about 7 atm to more than 10 atm in head lamps of automobiles and at about 20 atm in xenon lamps inevitably require the use of a high drive voltage that is much higher than 10 kV. Therefore, the drive power source of such a lamp is designed to generate a high starting voltage higher than 20 kV. Such a high voltage raises the manufacturing cost of the drive circuit because various components of the arc tube and the harness connected to it are required to show a high degree of dielectric strength. Additionally, noises can be generated at and near the base to give troubles to external facilities. Furthermore, if an electric discharge lamp requires a high starting voltage to energize starter gas for lighting, the voltage necessary for restarting the lamp is inevitably also high.
While efforts have been made to reduce the starting voltage of electric discharge lamps, no effective way of significantly reducing the starting voltage has so far been found.
In view of the above identified circumstances, it is therefore an object of the present invention to provide a lamp structure and a lighting method that can remarkably reduce the starting voltage of a high pressure electric discharge lamp in order to reduce the cost of the drive unit and alleviate the rigorous requirements for dielectric strength of the various components.
Another object of the present invention is to provide a high pressure electric discharge lamp that is free from fluctuations in the starting voltage that can typically occur as a function of the time consumed for lighting. As the starting voltage of high pressure electric discharge lamps is stabilized, lighting failures and ineffective lighting performances will be reduced to improve the manufacturing yield.
According to an aspect of the invention, in a high pressure electric discharge lamp including an arc tube having a discharge space containing at least a rare gas, a pair of electrodes projecting into the discharge space and arranged oppositely relative to each other, current feeding conductors for feeding the electrodes with an electric current, sealing portions extending from the arc tube and airtightly sealing the current feeding conductors, and an outer tube enclosing the arc tube, the outer tube can be airtightly sealed to the sealing portion, and gas, being apt to produce dielectric barrier discharges, is contained in a space surrounded by the outer tube and the arc tube.
According to another aspect of the invention, the gas apt to produce dielectric barrier discharges can be selected from the group consisting of Ne, Ar, Kr, Xe, F2, Cl2, Br2, I2, N2 and a mixture thereof.
According to another aspect of the invention, a pressure of said gas apt to produce dielectric barrier discharges can be not lower than 1.3 [kPa] and not higher than 100 [kPa].
According to another aspect of the invention a pressure of the gas contained in the space between the outer tube and the arc tube can be not lower than 40 [kPa] and not higher than 80 [kPa].
According to another aspect of the invention, the arc tube can be made of a material containing a dielectric substance. According to another aspect of the invention the discharge space of the arc tube does not contain mercury. According to another aspect of the invention, a method of starting a lighting operation of the high pressure electric discharge lamp is characterized in that the method includes producing a dielectric barrier discharge in the space surrounded by said outer tube and said arc tube by applying an electric field to said space from said current feeding conductors by way of dielectric used for said sealing sections.
According to another aspect of the invention, one of the electrodes projecting into the discharge space of the arc tube can be caused to discharge electrons from the surface thereof, in accordance with incidence of light generated by the dielectric barrier discharge into the surface of the electrode.
According to another aspect of the invention, an electric discharge can be started by inducing an electron avalanche, using the electrons as initial electrons. According to another aspect of the invention, the rated power of the discharge lamp is 35W. According to another aspect of the invention, the discharge space of the arc tube contains at least one metal halide.
According to another aspect of the invention, a method of starting a lighting operation of a high pressure electric discharge lamp is characterized by airtightly sealing an arc tube and an outer tube to form a space between them, producing a dielectric barrier discharge by applying an electric field to said space, and causing electrons generated by said dielectric barrier discharge to be transmitted through the discharge space of said arc tube and starting an electric discharge in said discharge space by the photoelectric effect.